From Sensory Substitution to Perceptual Supplementation
Résumé
The simple, almost naïve idea of sensory substitution systems is that of replacing a deficient sensory modality with another that is still available. The emblematic example of this approach is the TVSS (Tactile Vision Substitution System) designed for blind persons. Created in the late 1960s by Paul Bach-y-Rita, this device converts visual information captured by a video-camera into tactile signals in the form of a 20 × 20 matrix of points. In this way the image, reduced to 400 pixels (black or white without any intermediate levels of gray) is projected onto the skin by means of electromagnetic (or piezoelectric) point vibrators, or directly by electrical stimuli (Figure 58.1). If learning occurs while the camera is placed on a table and remains immobile, the perceptual capacities remain very limited and amount merely to patterns of tactile stimulation that are felt on the skin. However, if the blind person is allowed to grasp the camera and to explore simple scenes, then a significant change occurs: progressively (after something like fifteen hours of learning), the subject becomes able to recognize highly complex forms, even faces, and to locate these objects in the environment (Bach- y- Rita and Kercel 2003). Indeed, this increased capacity to recognize shapes seems to be accompanied by an externalization of the percepts (Epstein 1986; Auvray et al. 2005). The sensation of a succession of rapidly changing tactile stimuli on the skin, produced by the continual rotation and movement of the camera, drops out of consciousness; and is replaced by the perception of stable objects that are situated at a distance, “out there” in front of him (Bach- y- Rita 2004). This striking result raises a whole host of important questions, to be considered in this chapter, which are at one and the same time scientific, technological, and philosophical.